Replace RewardAccount with StakeAddress #163
Description
The definition of PoolRegistration and GovernanceProposal both use RewardAccount (which is a Vec) to hold the account that deposits should be returned to. This is held as the binary encoding of the address, with the 'e1' header (usually - I'm not sure whether script addresses can be used as well?).
This means there are several places where we need to extract the hash to match with other uses (e.g. in AccountState), which means reconstructing the StakeAddress from binary and then using get_hash().
It would be better if we stored the accounts as StakeAddress to begin with, either by redefining RewardAccount or replacing it.